The invention relates to direct contact steam injection heaters that use full pressure steam. In particular, the invention relates to a direct contact steam injection heater for heating certain types of slurries which contain material that tends to flocculate.
In direct contact steam injection heaters, steam is directly mixed into a flowing stock (e.g. liquid or slurry) being heated. Direct contact steam injection heaters are very effective at transferring heat energy to the flowing stock. They provide rapid heat transfer with virtually no heat loss to the atmosphere, and also transfer both the latent and the available sensible heat of the steam to the liquid.
The present invention was developed during ongoing developmental efforts by the assignee in the field of direct contact steam injection heaters. U.S. Pat. No. 5,622,655 entitled xe2x80x9cSanitary Direct Contact Steam Injection Heater And Methodxe2x80x9d by Bruce A. Cincotta et al., issuing on Apr. 22, 1997, and U.S. Pat. No. 5,842,497 entitled xe2x80x9cAdjustable Direct Contact Steam Injection Heaterxe2x80x9d, by Brian Drifka and Bruce A. Cincotta, issuing on Dec. 1, 1998, represent some of the prior developments in direct contact steam injection heaters by the assignee, and are hereby incorporated by reference.
These types of direct contact steam injection heaters use full pressure steam (i.e. the full amount of steam pressure available), and modulate the amount of steam added to the flowing liquid or slurry by a nozzle and plug configuration. The steam exits through the nozzle under sonic choked flow conditions. The high speed steam from the nozzle shears the flowing liquid or slurry, and creates a homogeneous blend in a combining region located downstream of the nozzle. As heat is transferred, the steam condenses.
Another direct contact steam injection heater was developed by the assignee for heating purified water or other liquids in which steam bubbles tend to merge to create large steam bubbles prior to condensing. This direct contact steam injection heater is disclosed in U.S. patent application Ser. No. 09/112,499, entitled xe2x80x9cDirect Contact Steam Injection Heaterxe2x80x9d, allowed Feb. 1, 2000, now U.S. Pat. No. 6,082,712, and incorporated herein by reference. This direct contact steam injection heater employs a Mach diffuser. The Mach diffuser injects a sonic velocity steam into the liquid stock through a plurality of relatively small steam diffusion holes. The Mach diffuser is generally coaxial with the heater body and resides within the inlet of a combining region. The purified water or other liquid flows in a radial direction through the inlet into the combining region and turns at an essentially right angle to flow through the combining region. The steam exits the coaxial Mach diffuser as small jets of steam injecting partially into the axial flow through the combining region. The velocity of the liquid flowing through the channel between the Mach diffuser and the combining region is maintained at a relatively high velocity (i.e., a relatively small flow area in the channel compared to the downstream portion of the combining region).
Although direct contact steam injection heaters are efficient and effective, stocks containing materials that flocculate tend to plug the heater if forced through bends and turns. Large flows of viscous stock cannot flow easily through the 90xc2x0 turns presented by certain prior art devices. For example, the direct contact steam injection heaters with adjustably positionable combining tube are not well suited for certain applications because the adjustable combining tube introduces additional creases, folds and pockets into which flocculating materials can accumulate. In addition, many designs are not easy to disassemble for manual cleaning.
Further, large volume flows of viscous slurries are difficult to heat with prior art direct contact steam injection heaters.
The invention is a direct contact steam injection heater in which steam is introduced into a flow of stock that is flowing axially through a pipe. The heater is installed in line and allows continued axial flow of the stock so the stock flow is not required to negotiate sharp turns when passing through the heater. That is, the heater includes a heater body having a flowing stock inlet and a heated stock discharge outlet that are aligned to provide axial flow through the pipe and the heater body. Full pressure steam is introduced into the stock through a Mach diffuser that is mounted transverse to the axial flow through the heater body. The Mach diffuser has a plurality of steam diffusion holes through which small jets of steam are injected into the flowing stock. The small steam jets break apart easily in viscous slurries and disperse before the steam has a chance to conglomerate into large bubbles which can create xe2x80x9csteam hammersxe2x80x9d and lead to unwanted vibration within the heating system. Furthermore, small steam bubbles dissipate heat more efficiently and thereby prevent hot and cold spots in the flowing stock.
The Mach diffuser has an adjustably positionable cover. The adjustably positionable cover obstructs a selected amount of the steam diffusion holes in order to modulate the amount of steam discharged through the Mach diffuser into the flow of stock. The cover is preferably rotatable relative to the longitudinal axis of the transversely mounted Mach diffuser.
The Mach diffuser preferably has a cylindrical wall containing the steam diffusion holes. The cover is preferably a cylindrical wall nested inside the cylindrical wall of the Mach diffuser, although if desired the cover can be placed on the outside of the cylindrical wall. The preferred cover has an internal region within the cylindrical wall that receives steam passing into the heater. The cylindrical wall has at least one steam opening that enables steam to flow from the internal region in the cover through the exposed steam diffusion holes in the Mach diffuser and into the axial flow of stock. Preferably, there are two steam openings in the cover, each consisting of a longitudinal slot located on opposite sides of the cover. The longitudinal slots preferably have widths that substantially occupy one quarter of the circumference of the cylindrical wall of the cover.
The Mach diffuser has an upstream surface area and a downstream surface area on its cylindrical wall, each occupying substantially one quarter of the circumference of the transversely mounted Mach diffuser. The upstream and downstream surface areas do not contain steam diffusion holes. The side surface areas on the Mach diffuser contain the steam diffusion holes. The Mach diffuser is oriented in the heater body so that the upstream surface area faces into the axial flow of the stock. This orientation to prevents unnecessary plugging of the diffusion holes on the upstream surface. Preferably, a deflector is mounted upstream of the Mach diffuser. The deflector deflects the flow of stock around the upstream surface area on the Mach diffuser and towards the side surfaces of the Mach diffuser. This prevents flow directly into a fluid stagnation point on the upstream surface of the Mach diffuser. The deflector is preferably welded to the inside wall of the heater body so that it does not become dislodged in the face of heavy flows of viscous slurries.
As mentioned, the downstream surface does not contain steam diffusion holes. This configuration helps to prevent the unnecessary formation of large steam bubble conglomerations. Large steam bubble conglomerations would likely be generated if steam diffusion holes were present on the downstream surface area because flow around the transversely mounted Mach diffuser normally separates from the cylindrical surface on the downstream side of the Mach diffuser.
The amount of full pressure steam discharged through the Mach diffuser into the axially flowing stock is modulated by adjusting the position of the cover over a selected amount of steam diffusion holes. This adjustment is preferably accomplished electronically with a rotating actuator having a key that engages one end of the cover. When the actuator rotates the key, the cover is positioned to expose a generally proportional amount of steam diffusion holes in the Mach diffuser. Radial jets of steam then flow through the exposed steam diffusion holes into the axial flow of stock.
It should be apparent to those skilled in the art that the use of an actuator to rotate the cylindrical cover for the Mach diffuser is especially accommodating for large volume flows through pipes having relatively large diameters. The rotatable cover allows for generally consistent injection of steam across the entire length of the transversely mounted Mach diffuser. In addition, the stroke on a linear actuator may create installation problems.
Various other features, objects, and advantages of the invention will be made apparent from the following detailed description and the drawings.